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Is molecular gas necessary for star formation?
Astrophysics of Galaxies · Submitted 16 May 2011
abstract compiled · 2026-09-23 · no language-model review
The authors' main claim We find that presence of molecules in the gas has little effect on the ability of the gas to form stars: star formation can occur just as easily in atomic gas as in molecular gas.
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The abstract, sentence by sentence
On galactic scales, the surface density of star formation appears to be well correlated with the surface density of molecular gas. This has lead many authors to suggest that there exists a causal relationship between the chemical state of the gas and its ability to form stars -- in other words, the assumption that the gas must be molecular before star formation can occur. We test this hypothesis by modelling star formation within a dense cloud of gas with properties similar to a small molecular cloud using a series of different models of the chemistry, ranging from one in which the formation of molecules is not followed and the gas is assumed to remain atomic throughout, to one that tracks the formation of both H2 and CO. We find that presence of molecules in the gas has little effect on the ability of the gas to form stars: star formation can occur just as easily in atomic gas as in molecular gas. At low densities (< 10^4 cm^-3), the gas is able to cool via C+ fine-structure emission almost as efficiently as via CO rotational line emission, while at higher densities, the main cooling process involves the transfer of energy from gas to dust, meaning that the presence of molecules is again unimportant. Cooling by H2 is particularly inefficient, accounting for as little as 1 percent of the overall cooling in the cloud. Rather than the chemical makeup, we find that the most important factor controlling the rate of star formation is the ability of the gas to shield itself from the interstellar radiation field. As this is also a prerequisite for the survival of molecules within the gas, our results support a picture in which molecule formation and the formation of cold gas are both correlated with the column density of the cloud -- and thus its ability to shield itself -- rather than being directly correlated with each other.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | On galactic scales, the surface density of star formation appears to be well correlated with the surface density of molecular gas. | background 1.00 | 0.10 | 0.02 | asserted 0.72 |
| s1 | This has lead many authors to suggest that there exists a causal relationship between the chemical state of the gas and its ability to form stars -- in other words, the assumption that the gas must be molecular before star formation can occur. | background 0.99 | 0.03 | 0.02 | hedged possibility 0.92 |
| s2 | We test this hypothesis by modelling star formation within a dense cloud of gas with properties similar to a small molecular cloud using a series of different models of the chemistry, ranging from one in which the formation of molecules is not followed and the gas is assumed to remain atomic throughout, to one that tracks the formation of both H2 and CO. | method 0.93 | 0.89 | 0.02 | asserted 0.79 |
| s3 | We find that presence of molecules in the gas has little effect on the ability of the gas to form stars: star formation can occur just as easily in atomic gas as in molecular gas. | result 1.00 | 0.97 | 0.88 | asserted 1.00 |
| s4 | At low densities (< 10^4 cm^-3), the gas is able to cool via C+ fine-structure emission almost as efficiently as via CO rotational line emission, while at higher densities, the main cooling process involves the transfer of energy from gas to dust, meaning that the presence of molecules is again unimportant. | result 0.68 | 0.87 | 0.09 | asserted 0.98 |
| s5 | Cooling by H2 is particularly inefficient, accounting for as little as 1 percent of the overall cooling in the cloud. | result 0.98 | 0.85 | 0.04 | asserted 1.00 |
| s6 | Rather than the chemical makeup, we find that the most important factor controlling the rate of star formation is the ability of the gas to shield itself from the interstellar radiation field. | result 0.90 | 0.96 | 0.77 | asserted 1.00 |
| s7 | As this is also a prerequisite for the survival of molecules within the gas, our results support a picture in which molecule formation and the formation of cold gas are both correlated with the column density of the cloud -- and thus its ability to shield itself -- rather than being directly correlated with each other. | interpretation 1.00 | 0.85 | 0.39 | asserted 0.60 |
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The paper
Original abstract
On galactic scales, the surface density of star formation appears to be well correlated with the surface density of molecular gas. This has lead many authors to suggest that there exists a causal relationship between the chemical state of the gas and its ability to form stars -- in other words, the assumption that the gas must be molecular before star formation can occur. We test this hypothesis by modelling star formation within a dense cloud of gas with properties similar to a small molecular cloud using a series of different models of the chemistry, ranging from one in which the formation of molecules is not followed and the gas is assumed to remain atomic throughout, to one that tracks the formation of both H2 and CO. We find that presence of molecules in the gas has little effect on the ability of the gas to form stars: star formation can occur just as easily in atomic gas as in molecular gas. At low densities (< 10^4 cm^-3), the gas is able to cool via C+ fine-structure emission almost as efficiently as via CO rotational line emission, while at higher densities, the main cooling process involves the transfer of energy from gas to dust, meaning that the presence of molecules is again unimportant. Cooling by H2 is particularly inefficient, accounting for as little as 1 percent of the overall cooling in the cloud. Rather than the chemical makeup, we find that the most important factor controlling the rate of star formation is the ability of the gas to shield itself from the interstellar radiation field. As this is also a prerequisite for the survival of molecules within the gas, our results support a picture in which molecule formation and the formation of cold gas are both correlated with the column density of the cloud -- and thus its ability to shield itself -- rather than being directly correlated with each other.
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References
How later papers used it
72 later papers cite it
3 later papers by other authors are listed below: 3 used its method or tool. 2 more papers by the same authors are listed: 1 used its method or tool and 1 used its data or values.
Uses its method or tool 4 of 11 papers with this label · 1 by the same authors
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We use the simplest molecular fraction model, in which the fraction solely depends on the density and temperature of the gas [ref] , and the dust-to-metals ratio is a smooth function based on the dust temperature.
The Star Clusters As Links between galaxy Evolution and Star formation (SCALES) project I: Numerical method (2024) · The hydrodynamical code GIZMO · 0.99
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To model the multiphase ISM, we self-consistently model the non-equilibrium formation and destruction of molecular hydrogen and CO, following the cooling and chemistry networks developed in [ref] and [ref] , where we also account for the self-shielding of molecular gas based on a column density computation on a healpix sphere.
On the impact of runaway stars on dwarf galaxies with resolved interstellar medium (2022) · Cooling, heating and star formation · 1.00
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Used a cooling model that another work introduced and this paper refined.
Properties of molecular clumps and cores in colliding magnetized flows (2022) · 0.99 · paraphrased: the licence does not allow quoting
By the same authors
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The thermal and chemical evolution of the gas are solved simultaneously, using the NL97 chemical network described in [ref] and the atomic and molecular cooling function described in [ref] .
Cloud properties across spatial scales in simulations of the interstellar medium (2024) (same authors) · M51: Full galaxy with Arepo · 0.99
Uses its data or values 1 paper · 1 by the same authors
By the same authors
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Used a typical CO clump brightness of 15 K from this paper to estimate a beam filling factor for CO-emitting clumps of less than 33 per cent.
New ALMA constraints on the star-forming ISM at low metallicity: A 50 pc view of the blue compact dwarf galaxy SBS0335-052 (2017) (same authors) · 0.94 · paraphrased: the licence does not allow quoting
How the literature uses it
9 arXiv papers cite this in 11 sentences
Papers citing it · 7 sentences without a confident label
Cites it as support 3 papers
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The distinction is important because it is not possible to produce Jeans masses as low as $∼M_⊙$ unless the gas can cool to $∼10$ K temperatures.] The subsequent numerical simulations of [ref] confirm this picture.
Metallicity-Dependent quenching of Star Formation at High Redshift in Small Galaxies · Introduction · 0.99
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However, we know from previous work [ref] that the behaviour of the gas is not particularly sensitive to whether the dominant coolant is C$^+$ or CO.
Modeling H2 formation in the turbulent ISM: Solenoidal versus compressive turbulent forcing · Chemistry and cooling · 0.97
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However, the difference between 20 K and 10 K can only slightly affect the star formation rate, as demonstrated by the recent simulations of [ref] .
The Abundance of Molecular Hydrogen and its Correlation with Midplane Pressure in Galaxies: Non-Equilibrium, Turbulent, Chemical Models · Introduction · 0.96
Cites it as background 1 paper
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For instance, it is not clear to what extent molecular gas is a prerequisite for star formation as opposed to being a by-product of the cold and dense gas phase normally associated with star formation [ref] .
Kennicutt-Schmidt relation of galaxies over 13 billion years in the COLIBRE hydrodynamical simulations · Introduction · 0.89
What it cites · 3 sentences without a confident label
Uses the method or tool of 1 paper
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Our modifications include a sink particle algorithm for treating gravitationally collapsing regions that become to small to resolve [ref] , the inclusion into the equation of motion of an optional confining pressure term [ref] , a treatment of gas-phase chemistry (described in more detail below), as well as radiative heating and cooling from a number of atomic and molecular species [ref] , and an approximate treatment of the attenuation of the Galactic interstellar radiation field (ISRF).
Approximations for modelling CO chemistry in GMCs: a comparison of approaches · Numerical approach · 1.00
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